This invention relates to a method of preparing coffee in which hot water flows through the coffee grinds. This invention further relates to a coffee maker for implementing this method.
Typically, coffee is prepared using devices comprising a water reservoir and an electric flow-through heater, in which heated water is fed down into a filtering device filled with the coffee grinds for extraction. The problem encountered with this method is that the amount of water of about 80 ml initially heated and delivered does not, as a rule, reach a temperature of 80.degree. C., As a result, the relatively cold water is able to wet the surface of the coffee bed only poorly with equally poor extraction results, passing through the filtering device into the collection container nearly unchanged. In cases where major batches of coffee are being prepared, this initially weak and barely aromatic extract can be balanced out without significant loss of flavor by the subsequent large amount of extract which has an intensive aroma because of sufficiently hot water and a sufficiently long extraction interval. By contrast, however, when only minor batches as, for example, two or three cups of coffee, are being prepared, experts and amateurs alike tend to characterize this beverage as rather flat in aroma. The reason for this is that in the preparation of small batches the extraction interval is relatively brief, causing sparingly soluble substances as, for example, acids which contribute to enhancing the aroma significantly to go into solution not until after a lengthy extraction period, as a result of which the preparation of small batches allows only part of the aroma to unfold.
To avoid this disadvantage, it is known in the art to prolong the extraction interval in coffee making by interrupting the supply of hot water to the filtering device from time to time. An automatic coffee maker known from DE-AS 11 66 988 which employs a mesh filter receiving the ground coffee provides for a continuous supply of hot water, with an electromagnetically operated valve being inserted in the discharge conduit leading to the mesh filter for enabling the discharge of hot water to be interrupted from time to time. The switch controlling the electromagnetic valve is actuated through two cam disks seated on a shaft which is rotated by means of a motor. The disadvantage of this known automatic machine is that it requires a motor for driving the camshaft, and further that its high-temperature water heater is constantly exposed to steam pressure; both the motor with the camshaft and the high-temperature water heater with its necessary safety devices are costly units which is the reason why this automatic machine is only suitable for commercial use.
Further, from DE-AS 20 58 824 a device for the preparation of coffee is known in which the main heating element can be set to a selected off-temperature by means of a scale on the thermostat corresponding to a preselected amount of brewing water, and can be turned off automatically by thermostat response for specified delivery pauses, with an approximately constant total brewing time for each preselected amount of brewing water. In this coffee maker, the water supplied to the brew vessel is invariably maintained in contact with the coffee bed for about the same length of time, independent of the amount of coffee being prepared, so that a strong coffee with an intensive flavor is obtainable for any amount.
Still further, an espresso maker is known from DE-A-42 40 429 in which a device for preheating the filter centrifuge is provided. In this arrangement, a valve controlling the passage of water is inserted upstream from the heated flow-through heater. Use of the reduced sectional area of orifice of the valve produces steam to heat the filter centrifuge, whilst use of the full sectional area of orifice causes hot water to be produced for brewing coffee. To heat the filter centrifuge, steam is initially directed through a riser to a collection container whence it is introduced through a vent pipe into the housing surrounding the filter centrifuge. As this occurs, the rotary container holding the coffee grinds is heated from outside to prevent the coffee beverage brought into contact with the preheated shell of the rotary container during centrifuging from cooling too rapidly.
Finally, from DE-28 11 497 C2 a coffee maker is known in which, in order to generate steam, first an amount of water per unit of time smaller than the amount subsequently needed for the brewing cycle proper is removed from the water reservoir. Accordingly, during presteaming the outlet valve is in a first position in which its sectional area of orifice for passage of the water is only small. As a result, such a low amount of water is allowed to exit as it evaporates completely in the flow-through heater, with a continuous development of steam setting in. The steam travels through the riser to the filtering device filled with coffee grinds, contacting the coffee grinds. Subsequently, after the outlet valve is moved to a second position for commencing the brewing cycle in which hot water instead of steam is delivered to the coffee bed, the coffee grinds contacted with steam can be wetted with the hot water in a manner ensuring a speedy and effective extraction whereby a flavor-intensive hot beverage is obtained, in particular also in cases where small batches of coffee are prepared.